WASHINGTON
— Vermont Sen. James Jeffords's anticipated defection from the Republican Party would have a profound effect on the lawmaking machinery of the 107th Congress - and possibly on President Bush's ability to direct the agenda in Washington.

True, the Senate would still be finely balanced between Democrats and Republicans, meaning that party cohesiveness remains a big factor in how floor votes on legislation turn out.

And the House would remain in GOP hands, providing Mr. Bush friendly territory from which to advance his ideas.

"Things get tougher and slower now," says Christopher Arterton, a political scientist at George Washington University, "especially if ... antagonisms between the Republicans in the House and a Democratic-controlled Senate get out of control."

At time of writing yesterday, Senator Jeffords was planning a trip to Vermont, where he is expected to announce his intentions today. Widespread reports indicated that he would say he is shifting his party registration from Republican to independent, or perhaps even to Democrat.

But with so much at stake, it was still possible that pressure or incentives produced by top Republicans could keep Jeffords in the GOP ranks.

Arguably more important than any new party label would be who Jeffords decides to back for the Senate leadership.

The current Senate is split 50-50, with Republican control depending on Vice President Dick Cheney's tie-breaking vote. That's the slim margin by which Mississippi GOP Sen. Trent Lott lords over the chamber as majority leader.

If Jeffords switches to independent, he could still support Senator Lott, meaning there would be little change in the Senate status quo. But it he decides to vote for Democratic Sen. Tom Daschle of South Dakota as majority leader ... well, that's when things begin to get really interesting.

Committee shape at stake

It would mean that Senator Daschle, not Lott, would have the biggest voice in determining when votes are taken on the floor - or even if particular votes are taken at all. It would mean new, Democratic chairmen for all Senate committees. The proportion of Democrats to Republicans in committees would increase.

The bottom line: greatly increased uncertainty for the future of Bush's already-ambitious second-wave agenda. His energy plan and his proposed partial privatization of Social Security would face a tougher audience, for instance.

Presidential personnel nominations, in particular, might have a tougher time. Consider Bush's nominee to be solicitor general, Ted Olson. The Judiciary Committee split 9-to-9 on whether to confirm him. Yet Lott advanced the nomination to the floor for a full vote, as is the majority leader's right.

Daschle, if majority leader, would be unlikely to make a similar move.

"Now if [the vote] is 9-to-9, it dies in committee," says Stephen Hess, a political analyst at the Brookings Institution here.

If nothing else, a party switch by Jeffords could give Democrats a morale boost. They would regain control of a chamber of Congress for the first time since 1994, and end the Republicans' current House-Senate-presidency power monopoly.

But the effect of his move on the course of the country might be less dramatic than its effect on bragging rights in Washington watering holes. The Senate is less rule-bound than the House, and more prone to be derailed or held up by a single individual with an agenda. Prevailing on most issues would still mean lining up 61 Senate votes - the number needed to avoid a filibuster.

The Senate is a powerful institution, of course. But in political terms, control of the Oval Office, or the more machine-like House, arguably means more.

"Of the three [House, Senate, and presidency], this is the one that makes the least difference," says Christopher Arterton of George Washington University.

So why might Jeffords switch? For one thing, because it might be popular - or at least not unpopular - in his home state.

Cows and quirkiness

Vermont's brand of liberal yet self-reliant Republicanism is out of step with a GOP increasingly controlled by conservatives from the West and South. The state's politics are famously quirky. Its lone member of the House, Bernard Sanders, is an independent and former Socialist.

Furthermore, Democrats did all they could to lure Jeffords over. Reportedly, the Democrats promised him the chairmanship of the Environment and Public Works Committee if he threw power in the chamber their way.

But in the end, GOP overreaction to his hesitations about the size of Bush's tax cut might have pushed Jeffords out. In a move widely seen as a swipe, Jeffords was not invited to a White House ceremony that honored a Vermonter as National Teacher of the Year. There were also reports that the White House was opposing renewal of a legislative dairy compact that regulates milk prices in Vermont and other New England states.

"Jeffords didn't jump. He was pushed," says Marshall Wittmann, a political analyst at the Hudson Institute. "This defection is completely owing to White House hubris."

After all, Mr. Wittmann points out, Jeffords had stayed in the GOP caucus for 25 years as a House member, and later senator, despite views that often put him at odds with the party leadership.

Jeffords has long been a supporter of abortion rights, as well as of stronger environmental laws than many other Republicans felt comfortable with. He supports legal protection against discrimination for homosexuals, as well as gun control - one position that is sometimes at odds with the many hunters in Vermont.

Jeffords supported President Clinton's failed health plan. In perhaps the clearest demonstration of his maverick tendencies, he voted against both articles of impeachment brought against Mr. Clinton by the House in 1998.